Optical memory techniques using an optical disk having a pit-like pattern as a high-density and large-capacity information medium have been put to practical use, the application thereof expanding to digital audio disks, video disks, text file disks, and then to data files. The functions enabling highly reliable and successful recording/reproduction of information on/from the optical disk with a micro-focused light beam are generally classified into a condensing function of forming a micro-spot of a diffraction limit, a focus control (focus servo) function of the optical system, a tracking control function, and a pit signal (information signal) detection function.
Recent progress in optical system design technology and transition to shorter wavelengths in semiconductor lasers, which are the light beam sources, has resulted in increased recording density of optical disks. An approach to increasing the density includes the increase in numerical aperture (NA) of optical disk in a condensing optical system that finely condenses an optical beam on the optical disk. In this case, the problem is associated with the increase in amount of aberration generated by inclination (the so-called tilt) of optical axis. Where the NA is increased, the amount of aberration generated in response to the tilt increases. The thickness of optical disk substrate (base material thickness) is decreased to prevent such an increase.
In compact disks (CD), which are called first-generation optical disks, an infrared light beam having a wavelength λ3 (wavelength λ3 is 780 nm to 820 nm) and an objective lens with an NA of 0.45 are used and the base material thickness of the optical disk is 1.2 mm. In the DVD of the second generation, a red light beam having a wavelength λ2 (wavelength λ2 is 630 nm to 680 nm) and an objective lens with an NA of 0.6 are used and the base material thickness of the optical disk is 0.6 mm. In the optical disks of the third generation, a blue light beam having a wavelength λ1 (wavelength λ1 is 390 nm to 415 nm) and an objective lens with an NA of 0.85 are used and the base material thickness of the optical disk is 0.1 mm.
In the present detailed description of the invention, the substrate thickness (or base material thickness) is a thickness from the surface of the optical disk (or optical recording medium) onto which a light beam falls to the information recording surface where information has been recorded.
Thus, the base material thickness of high-density optical disks has been decreased. From the standpoint of cost efficiency and space occupied by the device, an optical information device is desired in which information can be recorded/reproduced on/from optical disks of different base material thickness or recording density. Therefore, an optical head device is necessary that is equipped with a condensing optical system capable of condensing a light beam to a diffraction limit on optical disks with different substrate thickness.
For example, Patent Literature 1 discloses an optical information device designed for compatible reproduction. Principal components of the optical information device described in Patent Literature 1 will be explained below, as the conventional example, in a simple manner with reference to FIG. 27. FIG. 27 shows part of the objective lens that is provided in the optical head device located in the optical information device and serving to condense a light beam at the desired position on the optical disk.
In the conventional example, a diffraction action is used to condense, without aberration, incident light beams of substantially different wavelengths, such as a red light beam and an infrared light beam, through different base material thicknesses. As for a portion in which the grating period (pitch) should be decreased to increase the diffraction angle, the grating depth is increased and the grating period is enlarged. in FIG. 27, the objective lens has a region R21 and a region R22. The depth HB of the sawtooth diffraction grating of the region R22 is set to be twice as large as the depth HA of the sawtooth diffraction grating of the region R21, and the period (pitch) of the sawtooth diffraction grating of the region R22 is set to be twice as large as the period of the sawtooth diffraction grating of the region R21. The fabrication of the diffraction grating of the region R22 is thus facilitated.
When the shape of diffraction grating is changed between a plurality of regions and light beams diffracted from the plurality of regions are condensed in one point, as in the conventional example, phases of light beams in the boundaries of adjacent regions should be matched.
In the configuration described in Patent Literature 1, the height of the diffraction grating is doubled, the period is also doubled, and the diffraction direction is matched in the region R21 and region R22. The phase of the apex C21 as a boundary matches the phase of the apex C22 when the difference in optical path length that is caused by the height HA of the diffraction grating, that is, the phase difference, is an integer multiple of 2π.
However, in the case of products produced in huge amounts, such as optical disks, the wavelength of light beam emitted from a semiconductor laser used as a light beam source has a spread of several nanometers. Further, the wavelength also changes depending on the difference in operation environment temperature. Thus, even when the wavelength shifts from the designed center, since the diffraction direction is determined by the relationship between the pitch of the diffraction grating and wavelength and changes in the diffraction direction are the same, no mismatch occurs between the regions, but the configuration of the conventional example does not guarantee that phases will match.
In the configuration shown in FIG. 27, light beams of matched phases are assumed to incident from the lower side of the sheet surface. As shown in FIG. 27, the phases are matched vertically at a position BA. In this configuration, where the zone below the sloped surface is assumed to be a glass material (glass or resin), rather than air, as the light beam propagates upward in the glass material that has a refractive index different from that of the air, a phase difference is generated between the light beam propagating in the glass material and the light beam propagating in the air under the effect of sawtooth-shaped diffraction grating. Where the range between the apex C20 and the apex C22 in FIG. 27 is considered, since the light beam practically does not propagate in the glass material in the vicinity of the apex C20, no phase difference is generated. By contrast, on the side close to the apex C20 in the vicinity of the apex C22, the propagation length of the light beam in the glass material is the largest and the phase difference between this light beam and the light beam propagating in the air is at a maximum. Where the wavelength changes, the phase difference changes proportionally to the wavelength, and within the range from the apex C20 to the apex C22, the variation amount of phase is at a maximum in the vicinity of the apex C21.
Since phase variation of the diffracted light beam is averaged within the range from the apex C20 to the apex C22, the phase difference in the region R21 is a phase difference obtained in the case in which the glass material is present as far as a position M211 in the up-down direction in FIG. 27. Likewise, the phase difference in the region R22 is a phase difference obtained in the case in which the glass material is present as far as a position M212 in the up-down direction in FIG. 27. Since the phase difference obtained in the case in which the glass material is present as far as the position M212 in the up-down direction in FIG. 27 is different from the phase difference obtained in the case in which the glass material is present as far as the position M211 in the up-down direction in FIG. 27, the variation amount in the case in which the wavelength varies is also different. The resultant problem is that when the wavelength varies, the light beam diffracted from the region R21 and the light beam diffracted from the region R22 have the same diffraction direction, but a phase shift occurs and aberration occurs when the light beam diffracted from the region R21 and the light beam diffracted from the region R22 are condensed.
Thus, in the conventional example, the problem associated with a phase shift caused by the deviation of light beam source wavelength from the designed value occurring when the shape of diffraction grating is different in each region and light beams diffracted from the regions are condensed in one point has not been recognized and measures aimed at the resolution of this problem have not been disclosed.